Of the Box

To avoid starving, this bat varies its heart rate from 1000 to 200 beats per minute

Bats in Panama live their lives on the edge of starvation. They fly all night in search of fig juice—burning precious fuel in the process—and if they fail, they can die. Now, ecologists have uncovered the secret of the bats’ success: They radically change their heart rates, revving them up like a human sprinter when flying, and damping them down like a long-distance runner when resting.

“These bats have found a way to function at both extremely high levels of energy and very low levels,” says Kyle Elliott, an ecophysiologist at McGill University in Montreal, Canada, who was not involved with the work. “This is very unusual in animals.” Most lower their heart rate so drastically only for extended periods of time, such as when they’re hibernating, he notes.

To conduct the study, researchers led by Teague O’Mara, an ecologist at the Max Planck Institute for Ornithology in Seewiesen, Germany, outfitted four Uroderma bilobatum bats—mouse-sized creatures that build “resting tents” out of giant leaves—with miniaturized heart monitors. Each night, he and his colleagues chased after the animals, crashing through the jungle holding big antennas that let them hear as the bat heartbeats sped up and slowed down.

The heart rates were quite high—between 791 and 1066 beats per minute—when they were flying. That’s not quite a record, as blue-throated hummingbirds can reach 1260 beats per minute, and Etruscan shrews top out at about 1500 beats per minute (humans peak at about 240 beats per minute). But it’s nonetheless impressive, because the bats are much bigger, with bigger hearts to empty and fill, says Charles Bishop, a zoologist specializing in animal genetics at Bangor University in the United Kingdom who was not involved with the work.

But the big surprise was that when the bats were resting, their hearts periodically slowed down sharply, O’Mara and colleagues report this week in eLife. Several times each hour, the bats lowered their already slowed heart rates from about 300 beats per minute down to 200 beats per minute for about 6 minutes. Over the course of a day this saves 10% of their daily energy budget, the researchers report. “This could be the difference between life and death over a season,” Bishop says.

To find out how quickly the bats burn energy, the team fed Uroderma in the lab agave nectar, which has a relatively high proportion of a different kind of carbon atom compared to fig juice. They then measured the bats’ breath to see how fast that carbon was converted to carbon dioxide. They also measured levels of the hormone cortisol, which affects energy expenditure.

The tests suggest just how close to the edge these bats live. It takes the animals only about 8 minutes to begin to burn up the agave nectar and breathe out those unusual carbon atoms, O’Mara reports. Moreover, that meal made half their fat reserves by the next day. “They need all that energy to make their muscles go so they can cover the distances they need,” O’Mara says. “If they don’t find food that day, they are going to be in big trouble.”

“The study does a great job of integrating the physiological measurements in the lab with what is happening in the wild,” Elliot says. “There are very, very few studies that do that.”

The ability of these animals to drastically slow their heart rate is remarkable, he adds. “One moment they are a Mercedes tearing up the race track and chewing through fuel, the next they are a Mazda 3 going down the highway with maximum fuel economy.”